In all of the known practical electrochemical systems involving chlorine, the chlorine is converted into gas somewhere in the system. This is true for both the reversible electrochemical systems such as the zinc/chlorine battery, and for non-reversible systems such as the sodium chloride chlorine-manufacturing cell.
In the zinc/chlorine battery described, e.g., in U.S. Pat. No. 3,713,888, as the battery is charged, the chlorine emerges from the cell as chlorine gas which is then dissolved in water or electrolyte and the latter is then cooled to form chlorine hydrate, in which form the chlorine is stored until the battery is discharged. When the battery is discharged, the chlorine hydrate is decomposed into chlorine gas and water, and the chlorine gas is then dissolved in the cell electrolyte in order to sustain the discharge.
In the sodium chloride chlorine-manufacturing process, chlorine gas formed is transferred out of the cell, dried and mechanically compressed and cooled into liquid form for storage and disposal.
The transition of the chlorine through the gas phase into another form in the reversing and non-reversing electrochemical systems consumes energy in an amount depending on the process steps involved in the transformation. In the zinc/chlorine-hydrate battery system, the hydrate formation requires removal of 18 kilocalories of heat per mole of chlorine stored and to remove this heat by refrigeration requires upwards of 6 kilocalories of mechanical work per mole of chlorine stored. The heat needed to decompose the chlorine hydrate for discharge of the battery can be supplied from the cell heat but some mechanical work is needed in order to transfer the heat to the hydrate, and dissolving the chlorine into the cell electrolyte also entails mechanical work because of the high dissolving rates required for adequate discharge. A total expenditure of upwards of 10 kilocalories of mechanical work per mole of chlorine stored is necessitated by the gas phase transition. In the sodium chloride chlorine-manufacturing process, the drying, compression and cooling of the chlorine gas to form liquid chlorine consumes upwards of 4 kilocalories of mechanical work per mole of chlorine liquified.
The energy consumed by the chlorine gas phase transition is particularly burdensome in the case of a reversible energy storage system because the transition energy is a direct inefficiency of the system. In the zinc/chlorine-chlorine hydrate system, the total mechanical energy expended because of the transition (for charge and discharge) amounts to at least 10% of the total energy stored. In other reversible systems, which are less energetic than zinc/chlorine, the transition for hydrate storage is even more onerous. For example, in a hydrogen/chlorine-hydrate system, the gas phase transition amounts to at least 15% of the energy stored.
In addition to the energy consumption, the electrochemical systems involving chlorine require a large amount of auxiliary equipment. For example, a typical chlorine hydrate system can include four pumps, two gas dissolvers, two heat exchangers, and a refrigeration system as auxiliary components, amounting to about 30% of the total system cost. In a typical sodium chloride chlorine manufacturing process, two pumps, two heat exchangers, and one compressor are used as auxiliary components and typically account for about 25% of large system cost.
It has now been discovered that the gas phase transition can be entirely avoided in both reversing and non-reversing electrochemical systems involving chlorine and that the energy consumption required by such transition can also be avoided. Further, with respect to reversing electrochemical energy storage systems involving chlorine, it has been found that additional energy savings result indirectly from avoiding the gas phase for reasons which will be explained below.
Liquifying the chlorine as it is being generated by employing a much higher than normal pressure under the operating conditions has been disclosed heretofore. For example, Marconi Pat. No. 1,377,722 and Pieper Pat. No. 456,843 achieve the necessary pressure either by forcing a compressed gas into the electrochemical cell or by allowing the off gases of the electroytic reaction to accumulate in a gas space within the cell until the necessary pressure is achieved.
Accordingly, it is the object of this invention to provide a gas phase free, liquid chlorine electrochemical system and apparatus therefore in which considerable savings in energy consumption, equipment cost, and avoidance of cumbersome auxiliary equipment can be substantially realized. This and other objects of this invention will become apparent to those skilled in the art from the following detailed description in which: